Over recent years, from the viewpoint of global environment conservation, a fuel cell that has excellent power generation efficiency and that does not produce carbon dioxide emissions has been in the progress of being developed. This fuel cell causes hydrogen and oxygen to react with each other to generate electricity. It has a fundamental sandwich-like structure and includes an electrolyte membrane (that is, an ion exchange membrane), two electrodes (that is, a fuel electrode and an air electrode), diffusion layers for hydrogen and oxygen (air), and two separators. Depending on the type of electrolyte used, there have been developed a phosphoric-acid fuel cell, a molten carbonate fuel cell, a solid-oxide fuel cell, an alkaline fuel cell, a proton-exchange membrane fuel cell, and the like.
Among the above-mentioned fuel cells, the proton-exchange membrane fuel cell has the following features, compared to the molten carbonate fuel cell and the phosphoric-acid fuel cell:
(1) Operating temperature is markedly low at about 80° C.
(2) A cell main body can be reduced in weight and size.
(3) Quick start-up is realized, and high fuel efficiency and output density are achieved.
Therefore, among fuel cells, the proton-exchange membrane fuel cell is the one currently receiving the most attention for use as an onboard power supply of an electric vehicle, and household use or use as a portable and compact dispersed power system (a stationary type compact electric generator).
The proton-exchange membrane fuel cell is based on a principle in which electricity is obtained from hydrogen and oxygen via a polymer membrane. As illustrated in FIG. 1, its structure includes a membrane-electrode assembly (MEA, having a thickness of several 10 to several 100 μm) 1 sandwiched by gas diffusion layers 2 and 3 such as carbon cloth and separators 4 and 5, respectively. The membrane-electrode assembly has, as a unit, a polymer membrane and an electrode material such as carbon black carrying a platinum-based catalyst on the front and back faces of the membrane. This structure is formed into a single constituent element (a so-called “single cell”) to generate an electromotive force between the separators 4 and 5. At that time, the gas diffusion layers are frequently integrated with the MEA. Several tens to several hundreds of such single cells are connected in series to constitute a fuel cell stack to be used.
The separators 4 and 5 function as partition walls dividing single cells and are also desired to function as:
(1) a conductive material carrying generated electrons, and
(2) flow paths of oxygen (air) and hydrogen (an air flow path 6 and a hydrogen flow path 7, respectively, in FIG. 1) and exhaust paths of generated water and exhaust gas (the air flow channel 6 and the hydrogen flow path 7, respectively, in FIG. 1).
Further, durability is supposed to be about 5,000 hours in fuel cells for automobiles and about 40,000 hours in stationary type fuel cells used as household use compact dispersed power systems.
Proton-exchange membrane fuel cells that have been put into practical use so far have been supplied with a carbon material as a separator. However, the carbon separator has had disadvantages such as being liable to break upon impact, being difficult to reduce in size, and high manufacturing cost for flow path formation. In particular, the problem of high cost has been the largest obstacle in popularization of fuel cells. Therefore, there has been an attempt to apply a metal material, specifically, stainless steel instead of the carbon material.
As described above, the separator has a role as a conductive material that carries generated electrons and therefore needs to have conductivity. With respect to electric conductivity in use of stainless steel as the separator, since a contact resistance between the separator and the gas diffusion layer is dominant factor, some techniques to decrease the contact resistance have been investigated.
For example, Japanese Unexamined Patent Application Publication No. 2010-13684 discloses a technique in which stainless steel is subjected to immersion treatment in a solution containing fluorine ions at a rate of dissolution of 0.002 g/m2·sec or more and 0.05 g/m2·sec or less to incorporate fluorine into a passivation film on the surface to decrease contact resistance. This technique is effective in reducing the contact resistance. However, we conducted thorough investigations and found a problem that when stainless steel is dissolved by immersion treatment in a solution containing fluorine ions and then Fe ions having a concentration of 0.04 g/l (liter) or higher are mixed in the solution, the Fe ions and fluorine ions form a complex and thereby the effective amount of fluorine decreases, with the result that a predetermined effect may not be obtained. In other words, the following problems have been found: when steel sheets are treated, the number of treatable sheets is limited; and when a steel sheet in a coil is continuously treated, the treatable length is limited. Further, the following problem has been found: when this effect becomes weak, durability in a fuel cell usage environment markedly decreases.
Conventionally, as a method of maintaining the pickling power of a solution containing fluorine ions, for example, Japanese Unexamined Patent Application Publication No. 03-28386 (Japanese Patent No. 2827289) discloses a pickling treatment of metal in which the total iron concentration in a pickling solution is allowed to be 50 g/l or lower and a concentration ratio (Fe2+/Fe3+) of a divalent Fe ion (Fe2+) to a trivalent Fe ion (Fe3+) is controlled to be in the range of 0.25 to 2.0. However, since this technique is intended to simply maintain descale performance in a considerably high Fe ion concentration range (5 to 25 g/l in EXAMPLES), this issue largely differs from maintenance of an effect of advanced surface treatment so as to reduce contact resistance.
In view of the problems faced by conventional technology, that although immersion in a solution containing fluorine ions is effective in reducing the contact resistance of stainless steel, an effect of reducing the contact resistance may not be stably exhibited due to dissolution of the stainless steel itself being immersed in the solution, and in consideration of mass productivity, there is a need to provide a method of producing stainless steel having excellent conductivity and durability for use in a fuel cell separator, stainless steel for use in the fuel cell separator, a fuel cell separator, and a fuel cell.